As is well known, the entertainment industry is increasingly using small models and puppets in the area of special effects. To make these miniatures appear lifelike, it is often required that they be able to exhibit movement. To facilitate this, servomotors ("servos") are employed, which can range in size from 1- 1/2 inches square to 12 inches square and cost between $50.00 and $500.00 each.
It is often desirable to have these servos radio-controlled for aesthetic reasons (invisible to the camera), for portability, and to allow the puppeteer to move about freely. In these instances, a combination consisting of a transmitter, a receiver, and a servo are employed--which can be purchased as a pre-bundled set in a multiplicity of configurations (2 channel, 4 channel, or 8 channel). In addition, servos can-be purchased separately, as the quantity of them and sizes are dictated by the space available in which they are to go, and by the amount of movement needed for the miniature to appear realistic.
Radio-controlled servos are generally used by hobbyists in model planes, cars, and boats, and the transmitters/receivers are designed with that in mind. As is understood, the requirements of a steering mechanism for them is that the operation be linear in order that the movement of the joystick or wheel on the transmitter and the resulting response it produces remains the same over the entire range of control. Manufacturers, knowing this prerequisite, design their radio-controlled receivers to output a current with a pulse-width of between 1 and 2 milliseconds, which translates into a range of motion from 45 degrees left of the servo's quiescent state (at 1 millisecond pulse width), to 45 degrees right of the servo's quiescent state (2 millisecond pulse width). This thus results in a 90 degree rotation limitation, imposed, because servos become very significantly nonlinear in operation when they exceed 45 degrees rotation to either side of quiescence.
Technicians in the special effects industry often have occasions, however, where more than 90 degrees of rotation would be desirable--and have recognized that the servos themselves have a physical limit of rotation far in excess of 90 degrees (for example, the Futaba FPS 148 servo has a range of approximately 80 degrees left of quiescence and approximately 110 degrees right of quiescence for a total range of 190 degrees). But, due to the limitation of the typical radio-control, those extra degrees of rotation are possible only by a hard-wired servo driver card. On those occasions where hard-wiring is not acceptable, special effects technicians who need the extra movement generally achieve it by extending the arm of the servo to travel a further distance, although the servo does not rotate any further than the permitted 45 degrees in either direction. This method of obtaining additional movement has proven, unacceptable for four reasons; first, extending the arm of the servo takes up additional space which may not be available in smaller miniatures; second, every time the length of the servo's arm is doubled, there is a halving of the torque; thirdly, there is an increased probability of servo burnout, requiring replacement at a cost of $50.00 to $500.00; and fourthly, if a servo burns out, the miniature has to be taken apart for the servo to be removed and replaced, which frequently causes a significant amount of production down time--which, itself can cost thousands of dollars in overruns.